Patent GB-1352056-A (GB)
Title: Voltage level shift circuits
IPC: H03H/
Filed: 1971-04-27
Abstract: 1352056 Transistor voltage level shift circuits WESTERN ELECTRIC CO Inc 27 April 1971 [27 April 1970 17 Aug 1970] 11540/71 Heading H3T In a D.C. voltage level shifting circuit including a pass transistor 1 having its collectoremitter path coupled between a signal input terminal 10 and a signal output terminal 12 and having its base coupled to a point 11 of reference potential formed by the junction between a constant current source 2 and a resistor 21, the other end of the resistor 21 is connected to the input of the collector emitter path, so that, in operation, the base is controlled through the coupling to vary the conduction of the collectoremitter path of the pass transistor 1 to maintain a constant voltage shift in signals passing from the input terminal to the output terminal and also so that temperature changes affecting the pass transistor do not substantially affect the voltage shift. As shown in Fig.. 1 when all the transistors have the same characteristics if the temperature rises the base-to-emitter voltage V BE  of all the transistors decreases and the currents through the regulator resistor 22 and the resistor 21 increase. This produces an ncrease in the collector-base voltage which compensates for the decrease in base-emitter voltage of the pass transistor 1 to keep the level shift constant with temperature. If resistor 21 is the same as resistor 22 and the V BE 's are the same the level shift is equal to V s  so that the level shift may be controlled by changing V s  and is independent of the input voltage V IN . The circuit may be used to drive a transistor (31, Fig. 2, not shown) which is to operate at a voltage different from the input, and then V IN  is equal to V BE . If a diode is added in series with resistor 22 the driving point bias voltage will be zero. In a further embodiment (Fig. 3, not shown) a constant voltage level shift is produced between (101, 102) by controlling the conduction of the pass transistor (130) by a feedback circuit which includes a differential amplifier (110, 120). The output voltage is compared with the reference voltage produced at the junction of a resistor (141) and a constant current transistor (160) so as to obtain the constant voltage shift independently of temperature. The resistor (141) preferably has a temperature independent resistance characteristic or may be compensated by an electrical device having a complementary temperature controlled resistance characteristic connected in series with the resistor (141). The voltage level shift may be altered by altering the impedance of the resistor (141) or by altering the current through the transistor (160). In a modification (Fig. 4, not shown) which permits a wider range of adjustment of the voltage level shift a current by-pass including a transistor (250) and a resistor (291) transmit excess current directly to the output terminal (202) so as to prevent turn off of one of the differential amplifier transistors (210, 220). The effect of the parasitic base capacitance (267) of the transistor (220) on the voltage level shift is neutralized by the parasitic base capacitance (269) of the transistor (210). Two level shifting circuits of Fig. 3 can be symmetrically connected in the two sides of a balanced four terminal network (Fig. 5, not shown). The two level shifting circuits (375, 385) share a common constant current source (393) which is connected to two transistors (371 and 372) which enable independent adjustment of the voltage level shifts. The level shifting circuits can be constructed in integrated circuit form.